This invention relates to motor control and power systems. In particular, this invention relates to systems for providing substantial and enhanced power for the operation of motors in power-operated devices during load conditions and for providing boosts of additional power upon demand during load conditions. In addition, this invention further relates to systems for protecting such motors during overspeed conditions and for selectively applying dynamic braking upon shut down of motor operation. Also, this invention relates to methods of operating the power systems.
A universal motor includes a field winding or coil which is connected in series with a rotatable armature winding and can operate from either an A.C. or D.C. input. A permanent magnet motor, which is referred to as a PM motor, includes a rotatable armature winding and utilizes a permanent magnet to provide the magnetic field which is provided by the field coil in the universal motor. The PM motor also can operate from either a rectified A.C. or a D.C. input and functions similarly to a series motor. Both the universal and PM motors have excellent starting torque but experience drops in speed as the driven load increases which necessitates additional current from the power source.
Currently, universal and PM motors are used in a variety of power-operated devices such as, but not limited to, household and kitchen products, power tools and outdoor lawn and garden equipment. Typically, such power-operated devices include internally of the device a universal or PM motor which is powered by a conventional A.C. source. In some instances, the A.C. voltage of the source is rectified by a rectifier which is contained within the device to facilitate the application of a pulsating D.C. voltage to the motor. Since the D.C. voltage is pulsating at twice the frequency of the conventional A.C. source, torque pulsations occur at the same frequency and are mechanically transmitted throughout the motor which results in undesirable noise and vibration.
In addition, because the D.C. voltage is pulsating, the average D.C. voltage effectively applied to the motor is limited and thereby limits the effective power output and the speed of the motor. Also, since losses in the motor are proportional to the effective value (RMS) of the current, substantial losses are generated in the motor at heavy loads to deliver the required load. One way of increasing the power capability of the power-operated device is to increase the size of the motor to increase the cooling capability. However, this results in a heavier device and detracts from the portability and maneuverability of the device.
Thus, there is a need for a power-operated device which can improve the power output and speed capability of a motor of the device while not detracting from the portability of the device.
Frequently, a power-operated device which uses a universal or a PM motor encounters a load change which affects the speed of the motor. For example, a lawn mower which uses a universal or PM motor may be operating in an unloaded condition and then is moved into a normal grass cutting mode where the blades of the mower encounter load conditions such as a moderate height of grass. This results in a reduction of the speed of the motor. When the blades of the lawn mower encounter additional load conditions such as higher heights of grass, a significantly increased load is placed on the motor whereby the motor speed decreases further. During these periods of reduced speed, the motor is operating inefficiently and, if the additional load conditions continue, the mower may not recover and eventually will stall.
Several known motor control systems employ the principle of detecting changes in the counter EMF (CEMF) of the motor to indicate changes in the speed of the motor which typically result from changes in the motor load. In many of these systems, a silicon-controlled rectifier (SCR) is connected in series with the armature. The CEMF fluctuations are used directly or indirectly to control the firing angle of the SCR and, thereby, control the period of connecting the armature to line voltage. In such systems, the SCR functions as a rectifier and, typically, is conducting for less than a half of each full cycle of power source operation. Since the SCR conducts for less than a half cycle, the power delivered to the motor is comparatively low. At least one other system employs two SCR's to control the flow of current alternately in both directions through the motor with one SCR controlling low-to-medium speeds and the other SCR controlling medium-to-high speeds of the motor. Even so, operating power is applied to the motor during less than a full cycle of each cycle of operation of the power source. Examples of both of the foregoing systems are illustrated in U.S. Pat. No. 4,181,876 which issued on Jan. 1, 1980.
In any event, the above-noted SCR systems control the supply of current directly from the line as a means of speed control. These systems do not provide a sustained application of operating power to the motor with a power boost from a second source if needed when the motor is subjected to load conditions.
In another system which is illustrated in U.S. Pat. No. 3,588,653 which issued on June 28, 1971, the firing angle of an SCR, which is connected in series with the motor, is controlled independently of the motor CEMF to furnish power from a primary power source to the motor during appropriate periods. A capacitor is connected in parallel with the motor and is charged during conduction of the SCR and furnishes power to the motor when the SCR is not conducting. The system illustrated in U.S. Pat. No. 3,588,653 furnishes power from the primary source to the motor during periods when the SCR is conducting and furnishes power from the capacitor to the motor at times when the SCR is not conducting. Therefore, the system does not respond to load conditions wherein the motor requires a boost of power to overcome the apparent inability of the motor to handle such load conditions.
Thus, there is a need for a motor control and power system which not only furnishes power continuously from a primary power source to a motor but also furnishes a boost of power to the motor independently of the primary power source during periods when the motor is subjected to load conditions.
Some motor control systems employ various types of switching elements in the control of motor speed. If any of these elements fail such as by shorting, the motor could enter an overspeed mode culminating in damage to or destruction of the motor and any surrounding equipment.
Thus there is a need for a system which responds to such element failures and not only prevents the motor from entering an overspeed mode but also promptly brakes the motor.
In the use of universal motors, it is common practice at the time of braking the motor to reverse the connection of the field coil with the armature and to connect the coil and the armature, and optionally a braking resistor, in a series loop to provide a dynamic braking effect. This principle of dynamic braking is described as prior art in U.S. Pat. No. 4,144,482 which issued on Mar. 13, 1979.
When an A.C. line voltage is used as the direct power source for the motor, the voltage waveform passes through a zero level and then proceeds in the opposite direction. As the voltage waveform passes through the zero level, there is no current through the field coil to develop a field. However, there is sufficient residual magnetism in the field core to establish a field of low level flux. As the voltage waveform moves away from the zero level, a field in the opposite direction is developed which is expended to overcome the flux resulting from the residual magnetism. At that time, there is no field through which the armature can rotate and, hence, the field is at a zero flux level. This condition does not interfere with the normal operation of the motor. However, if a brake switch is closed at the time the field is at the zero flux level, dynamic braking will not occur because the armature must be rotating through a field of some flux in order to effect such braking.
Thus, there is a need for a dynamic braking circuit which overcomes the disadvantage of the zero flux effect noted above to insure that dynamic braking is available when needed. Also, there is a need for simple but unique braking circuits which quickly and effectively brake a motor in dynamic fashion.
In summary, there is a need for a power system which can provide, upon demand, a boost of additional power to a power-operated device when the device encounters load conditions. Further, there is a need for a power system which includes facility for preventing overspeed of the motor due to internal failings within the power system and also for providing dynamic braking of universal motors used with such power-operated devices.